Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesive which has been of considerable interest in recent years is the bonding of nonwoven fibrous material to a polyurethane substrate in articles such as disposable diapers, incontinence pads and similar articles.
One aspect of forming an appropriate bond between the non-woven layer and polyurethane substrate of a disposable diaper, for example, is to avoid loss of adhesive in the valleys or gaps formed in the irregular surface of the chopped fibrous or fluff-type material which forms the non-woven layer. If the adhesive is discharged onto the non-woven layer in droplet form, for example, a portion of the droplets can fall between the gaps in the surface of the fibrous, non-woven material. As a result, additional quantities of adhesive are required to obtain the desired bond strength between the polyurethane substrate and non-woven material.
This problem has been overcome by forming hot melt thermoplastic adhesives in elongated, thin strands or fibers which are deposited atop the nonwoven material and span the gaps in its irregular surface. Elongated strands or fibers of adhesive have been produced in spray devices which include a nozzle formed with an adhesive discharge opening and one or more air jet orifices through which a jet of air is ejected. A bead of adhesive is ejected from the adhesive discharge opening in the nozzle which is then impinged by the air jets to attenuate or stretch the adhesive bead forming a thin fiber for deposition onto the substrate. Examples of spray devices which are capable of dispensing a viscous material in the form of elongated strands or fibers are disclosed in U.S. Pat. No. 2,626,424 to Hawthorne, Jr.; U.S. Pat. No. 3,152,923 to Marshall et al; and, U.S. Pat. No. 4,185,981 to Ohsato et al.
In applications such as the formation of disposable diapers, it is important to carefully control the pattern of the adhesive fiber deposited onto the non-woven substrate in order to obtain the desired bond strength between the non-woven layer and polyurethane substrate using as little adhesive as possible. Improved control of the pattern of adhesive fibers has been obtained in dispensing devices of the type described above by impacting the adhesive bead discharged from the nozzle with air jets directed substantially tangent to the adhesive bead. The tangentially applied air jets control the motion of the elongated fiber of adhesive formed from the adhesive bead ejected from the adhesive discharge opening in the gun nozzle, and confine the elongated fiber in a relatively tight, or compact, spiral pattern for application onto the substrate. Structure which produces a spiral pattern for deposition onto a substrate is disclosed, for example, in the '424 Hawthorne, Jr. patent and the '981 Ohsato et al patent mentioned above.
In order to produce a compact spiral spray pattern of adhesive fibers in the dispensing devices described above, it is important to ensure that the air jets are directed tangentially relative to the bead of adhesive ejected from the nozzle of the dispensing device. This requires accurate placement of the bores or passageways through which pressurized air is ejected from the nozzle or gun body of the dispensing device, which are typically on the order of about 0.015 to 0.020 inches in diameter. The boring or drilling of passageways having such a small diameter at the appropriate angles in the nozzle and/or gun body of prior art dispensing devices is a relatively expensive and difficult machining operation.
Many problems with prior art adhesive fiber dispensing systems have been overcome by the nozzle attachment disclosed in U.S. Pat. No. 4,785,996, which is assigned to the same assignee as this invention. The nozzle attachment disclosed in U.S. Pat. No. 4,785,996 is adapted to mount to the nozzle of a standard adhesive gun which is formed with an adhesive discharge opening connected to an adhesive passageway in the gun body and an air discharge opening connected to an air passageway in the gun body. The nozzle attachment is a one-piece annular plate formed with a boss extending outwardly from a first surface of the plate and a nozzle tip extending outwardly from a second surface of the plate. A throughbore is formed between the boss and nozzle tip which communicates with the adhesive discharge opening in the nozzle of the gun body when the plate is mounted to the nozzle. Heated hot melt adhesive is transmitted through the adhesive passageway in the gun body, out its adhesive discharge opening and then into the throughbore in the plate. The adhesive is ejected as a bead through the nozzle tip toward a substrate.
The nozzle attachment is formed with an annular notch or groove which extends from its first surface having the boss toward the second surface formed with the nozzle tip. The annular groove is provided to assist in drilling air jet bores in the plate through which jets of pressurized air are directed at an angle of about 30.degree. and tangent to the adhesive bead ejected from the nozzle tip. One surface of the annular groove is oriented substantially perpendicular to the axis of movement of the drill bit, i.e., at an angle of about 30.degree. to the first and second surfaces of the plate, and sufficient clearance is provided within the annular groove to avoid interference with the drill bit. As a result, sliding of the drill bit relative to the plate is minimized during the drilling or boring operation which helps locate the air jets bores at the desired angle in the plate.
While the nozzle attachment disclosed in U.S. Pat. No. 4,785,996 solves many of the problems of prior art devices designed to spray viscous material such as hot melt adhesive, some deficiencies have been discovered in certain applications. It has been found that the formation of a groove in the relatively thin nozzle attachment or plate can result in deflection of the nozzle attachment during operation, particularly where the threaded mounting nut which mounts the nozzle attachment in place on the nozzle is secured with too much torque. This deflection can form a leakage path at the interface between the nozzle attachment and nozzle of the spray gun. In some instances, it has been found that hot melt adhesive entering the nozzle attachment has flowed along this leakage path and been deposited in the annular groove where the air flows into the air jet bores. This can clog the air jet bores and restrict the flow of air necessary to attenuate or stretch the adhesive bead to form adhesive fibers.
Another potential problem with the nozzle attachment of U.S. Pat. No. 4,785,996 is that the air jet bores are drilled in the plate or nozzle attachment from the inner side or surface which contacts the nozzle toward the outer side or surface formed with the nozzle tip. Because the air jet bores are so small in diameter, i.e., 0.015 to 0.020 inches, it is possible for the drill bit to drift or move off line in the course of passing through the nozzle attachment from its inner side to the outer side. As a result, the discharge outlet of the air jet bores at the outer side of the nozzle attachment might be slightly out of position and this can affect the efficiency of the nozzle attachment in forming adhesive fibers because the air jets may not impact the adhesive bead precisely tangentially thereto.
The nozzle tip of the nozzle attachment disclosed in U.S. Pat. No. 4,785,996 protrudes from the outer surface thereof when mounted to the nozzle of the gun body, and extends outwardly from the mounting nut which secures the nozzle attachment to the nozzle of the spray gun. A cavity or space is thus formed between the nozzle tip and such mounting nut. When the spray gun is operated intermittently, it has been found that cut-off drool, i.e., adhesive remaining after the gun is shut off, can collect in the space or cavity between the nozzle tip and mounting nut. This cut-off drool might collect and partially block the discharge outlet of the air jet bores formed in the nozzle attachment, thus affecting the performance of the nozzle attachment in forming adhesive fibers. Additionally, the protruding nozzle tip is exposed and can be damaged if it contacts the target substrate or other object during operation of the spray gun.